Electron discharge device with



Feb. 25, 1936. E, LEDERER ET AL Re. 19,870

ELECTRON DISCHARGEDEVICE WITH INDIRECTLY HEATED CATHODE Original Filed Nov. 16, 1927 INVENTORS EENE-ST ,4. 4 505552.

7 JOHN l4! M EN ATTORNEY Reissued Feb. 25, 1936 UNITED STATES PATENT OFFICE ELECTRON DISCHARGE DEVICE WITH INDIBECTLY HEATED CATHODE Ernest A. Lederer, Glen Ridge, and John W.

Marden, East Orange, N. J., assignors to Westinghouse Lamp Company, a corporation of Pennsylvania 3 Claims This invention relates to an electron discharge device and more particularly to such device of the type in which the cathode is heated indirectly by a heating element disposed adjacent thereto.

In the manufacture of electron discharge devices in which the cathode is designed to be heated through the agency of alternating current, it has been the usual practice to construct the cathode 1n the form of a hollow metal cylinder and to heat the same by conduction from an electrically insulated heating element. The heating element of such cathode usually consists of a tungsten filament which is supported within an aperture in a cylindrical insulator about which the hollow metal cylinder, coated with a thermionically active material such as the oxides of the alkaline earth metals, is positioned. This coated cylinder constitutes an equi-potential cathode and may be provided with a terminal separate from those of the heating element.

In constructing cathodes of this type, difficulty has been encountered hereiore, due to interaction of the tungsten filament with the insulator and to the fusing of the insulator onto the filament, resulting in early burn-out of the heating element. This difficulty has been overcome to a large extent by the use of thorium oxide, zirconium oxide or rare earth oxide in place of the usual porcelain insulator.

The method which is used very largely and which is preferred by us for producing the thermionically active coating on the metallic cylinder is to first coat the cylinder with the carbonates of the alkaline earth metals, which are r baked thereon in a firmly adherent manner in an atmosphere of carbon diordde. After the oathode has been assembled within the evacuated device, the cylinder is heated to a high temperature to convert the alkaline earth carbonates into oxides of the alkaline earth metals. It has not .been found advisable to effect this conversion of the carbonates to the oxide before assembly of the cathode in the device, since the oxides apparently become contaminated in air and the electron emission of the cathode is impaired and heretofore, this heating of the metal cylinder has been accomplished by passing a heavy current through the heating elements to raise the same to an abnormal temperature.

However, due to the high temperature to which the heating element must be raised to heat the surrounding cylinder to the decomposition temperature of the carbonates, a chemical reaction appears to take place between the tungsten and the insulator when the same is made of porcelain or isolantite, possibly due to interaction between the tungsten and water vapor, carbon dioxide or other gas liberated from the insulator. The insulator also fuses on to the tungsten heater wire during this heat treatment and upon cooling of the cathode assembly, a portion of the tungsten adheres to the porcelain or isolantite and is chipped off from the tungsten body, due to the difference in contraction of the insulator and the tungsten filament. As a result of these difiiculties, the filament burns out prematurely and the life of the device is relatively short.

One of the objects of the present invention is to provide a construction in which the above mentioned difficult'les will be avoided and in which conversion of the carbonate coating on the oathode may be effected without necessity of employing the enclosed heating element Another object is to provide a construction in which the use of solid insulating material, such as porcelain, between the cathode and the heating element may be entirely dispensed with.

A further object is to provide a construction in which the electrodes will be shielded from electrons emitted from all portions of the heating element.

A further object is to provide a simple and rigid support for the electrodes which will enable the cathode to be heated by high frequency induction current without substantial heating of the other electrodes.

Other objects and advantages will hereinafter appear.

In accordance with the present invention, we construct the cathode in the form of a hollow metallic member having a coating of a thermionically active material on the exterior thereof and a heating element contained therein. The heating element may be supported within a refractory insulating member contained within the cathode so as to heat the same by conduction or the refractory insulation may be entirely omitted and the cathode heated by radiation from the heating element. If desired, the heating element may be operated at an electron emitting temperature and a difference in potential may be maintained between the heating element and the cathode to cause the cathode to be heated by electron bombardment from the heating element.

The cathode is arranged so as to form one portion of a closed loop in which the currents may be induced by a high frequency coil disposed outside of the envelope. This closed loop is arranged in such position that, upon heating thereof by high frequency induction current, heating currents are not set up in other and undesired portions of the electrode assembly.

With this construction, it is possible to heat the cathode during exhaust, to a high temperature to decompose the carbonates of the alkaline earth metals thereon and to convert the same into oxides of the alkaline earth metals without heating the filament and having undesired reaction with carbon dioxide or other gases formed during the operation.

Shields are disposed about the upper and lower ends of the cathode to prevent stray or undesired discharges taking place between the heating element and the control electrode or anodewhich might result in alternating current hum when the device is employed inradio receiving apparatus.

In order that the invention may be more fully understood, reference will be had to the accompanying drawing in which:

Fig. 1 represents an electron discharge device partly in section embodying the present invention;

Fig. 2 is a sectional view taken on line IIII of Fig. 1;

Fig. 3 is a fragmentary view showing a modified form of cathode construction; and

Fig. 4 is a sectional view of a further modified form of cathode construction.

The electron discharge device shown inFlg. 1 comprises a hermetically sealed envelope I having an electrode assembly II therein composed of a cathode I2, grid or control electrode I3 and anode I4. The cathode I2 is in the form of a hollow metal cylinder, preferably of nickel having on the exterior surfacethereof a coating I5 of the oxides of the alkaline earth metals or other thermionically active material. Annular discs I6 and I1 surround the upper and lowerends respectively of the cathode I2 and are rigidly secured thereto by a number of radially extending supporting strips I8. The-discs I6 and I1 serve to support the cathode from the support wire I9 through intermediate supporting strips 2|] welded to the discs and to the support wire. The

support wire I9 at its lower end is secured to a rigid wire 2| sealed in the press 22 of the device, and joined to leading-in conductor 23. At the upper end the support wire I9 is welded to two bracing supports 24 and 25 secured in a glass bead 26 positioned above the electrode assembly.

A heating element 21 which may take the form of an inverted V-shape filament of tungsten or other highly refractory metal, having the adjacent legs arranged to neutralize the electric fields set up therein, is disposed within the cylindrical cathode I2 and is supported at its loop portion by a resilient member 28, having one end secured in the glass bead 26. The free ends of the filament are joined to support wires 29 and 30 which are connected to the leading-in wires 3| and 32 respectively for supplying heating current trons'from the exposed ends 43 and 44 of the heating element. F'We have found, due to the high temperature at which the heating element is operated, that electrons are emitted therefrom and drawn to the other electrodes if some shielding means is not provided. This electron flow sets up a fluctuating current in the output circuit of the device which, in many cases, is detrimental toits operation. The discs I6 and I! may be composed of metal or a suitable insulating ma- -terial-'such as mica. If tbey:are composedlofln- 'sulatingmaterial it is, of course, necessary to extend the supporting. strips 20 over to the hollow cylinder I2 to conduct the current tothecathode from the support wire I9. Obviously, in place of making the members I6 and I! of disc form,

they maybe conicalor any other suitable shape which willprovide a barrierbetween the exposed ends of the heating element and the anode and control electrode.

The supporting members 20, discs I6 and I1, support wire I9 and cylinder I2 form a closed loop. The cathode I2 should be formed of sheet metaland the remaining portions ofthe loop of sufiiciently heavy. material to render it possible to heat up the cathode cylinder to a high temperature by means of a high frequency induction coil disposed externally of the envelope and arranged with its axis substantially normal to the bulb. This permits the oxide coating on the cathode to be formed from a coating of the alkaline earth carbonates after the cathode is incorporated in the exhausted envelope without necessitating the heating of the tungsten-file ment 21.

Of course, if desired, the shields I6 and I1 4 may be omitted in which case the cathode cylinder I2 may be supported directly from the wire I9 by the supporting strips 45 and 46 as shown,

in Fig. 3 so as to form theclosed loop with the cathode. InFig. 4 a modified form or cathode is illustrated in which a solid insulating member 41 is positioned within the cathode I2 and has a pair of apertures therein through which the heating element 21 extends, whereby the cathode is heated by conduction rather than radiation. In the form shown in Figs. 1, 2 and'3 in which the insulating member 41 is not employed, the

cathode I2 maybe heated by electron bombard ment from the heating'element 21' by maintain- "ing the heating wire negatively chargedwith respect to the interior surface of the cylinder.

It is obvious, of course, that many changes may be made in the construction shown'and "de-' scribed and wedo'not desire to be limited there- 'by but desire the described embodiments to be construed in all respects as illustrative and'not restrictive, reference being had to the appended claims to indicate the scope of the invention.

.What is claimed is:

1. An electron discharge device comprising an evacuated envelope, a hollow thermionicallyactive uni-potential cathode-and ananode,--anelectronemitting filament'within said cathode having its opposite ends non-inductively arranged, said electron emitting filament extending beyond one limit of said cathode and means externally of said cathode for shielding said anode from the electrons emanating from said electron emitting filament.

2. An electron discharge device comprising an evacuated envelope, a hollow cathode therein, a heating element disposed within said cathode, ra dially extending dielectric shields disposed externally about the opposite ends of said cathode and. an anode positioned between said shields.

3. An electron discharge device comprising an evacuated envelope, a tubular cathode therein, a heating element within said cathode, a closed electrically conductive loop within said envelope including said cathode, whereby said cathode may be inductively heated from an external source, an annular dielectric shield surrounding each end of said cathode and a plurality of electrodes disposed about said cathode between said shields.

ERNEST A. 'LEDERER. JOHN w. MARDEN. 

